Field of the Invention
The present invention relates to a circuit and method of reducing the inrush current of an induction motor. The circuit is particularly useful in phase conversion applications such as to convert single phase alternating current conversion to three phase alternating current.
Description of the Prior Art
It is common in the art to use a single phase VAC supply to operate a three phase motor to generate three phase VAC power at the three phase motor terminals. A single phase VAC supply comprised of a hot leg and neutral conductor is coupled to at least two of the AC induction motor terminals, or a phase-shift capacitor may be used between the hot leg and the third AC induction motor terminal. When connected to the single phase VAC supply having a steady-state voltage and frequency the three phase motor will result in a significant inrush current drawn from the single phase VAC supply. If on the other hand, three phase VAC power is available, the three phase motor can be started and run directly from the three phase VAC power. Regardless of whether the motor is started from single phase or three phase power, motor start events are generally accompanied by a temporary current draw of anywhere from 5-8 times the normal current draw. This temporary current demand can trip breakers or temporarily cause the power from the utility company to dip below acceptable standards. Thus, it is desirable to reduce or avoid inrush current encountered during three phase motor start events.
One manner of reducing inrush currents at three phase motor start events comprises using a Variable or Ramped Frequency three phase VAC drive or supply (VFD) to start the three phase motor. The VFD is coupled to the three phase motor and the frequency of the inverter three phase output is gradually increased until the three phase motor reaches a steady-state condition or a motor working speed. The gradual start of the three phase motor eliminates or significantly reduces inrush currents.
As one example, U.S. Pat. Nos. 5,272,616, 5,402,053, and 5,969,957 to Divan et al. disclose a single phase to three phase converter circuit that discloses converting single phase to DC and switching or inverting the DC to create three phase power. The converter may be operated in a startup mode, which provides three phase VAC power at a low frequency to start the motor and a second mode to wherein single-phase power is used to power the three phase motor. Divan et al. however teach that the inverter circuit is circuit is used to create single phase power which causes unnecessary expense and wear and tear on the components only during three phase motor start events. The present invention overcomes this and other shortcomings of the prior art.